Hollow fiber membranes for a purpose of treatment of aqueous fluids have been widely utilized for industrial use such as microfiltration or ultrafiltration and for medical use such as hemodialysis, hemofiltration or blood diafiltration. Particularly in recent years, there has been a demand for a technology where pathogenic substances such as virus are removed from a solution of protein which is a useful ingredient during the steps for the manufacture of biopharmaceuticals and blood products so as to enhance the safety.
According to the non-patent document 1, it is said to be desirable, with regard to the steps of removal and inactivation of virus in a fractionated plasma preparation, to grapple with more than two different viral inactivation and removal steps. According to the description of the non-patent document 2, it is mentioned that the LRV to be achieved as the target value is about 4. Further, according to the non-patent document 3, there is a clear description in this document reading “Particularly with regard to the steps of removal and inactivation of virus, ‘it is desirable to consider in more than two different viral inactivation and removal steps’ in ‘About the guideline concerning the security of safety of fractionated plasma preparations against virus’ (Dispatch No. 1047 for Drugs (Aug. 30, 1999)) and, with regard to the specific virus, it is requested that the sum of virus clearance indexes in the manufacturing steps (total virus clearance indexes) is 9 or more.” Incidentally, the above term LRV has nearly the same meaning as the virus clearance index R which is mentioned as follows in the non-patent document 1.Virus clearance index R=log ((V1×T1)/(V2×T2))
V1 Volume before the treatment of the step
T1 Titer of virus before the treatment of the step
V2 Volume after the treatment of the step
T2 Titer of virus after the treatment of the step
As to a method for removal/inactivation of virus, there are a heating treatment, an optical treatment such as irradiation of gamma ray or ultraviolet ray, a chemical treatment such as a low-pH treatment, a precipitating fractionation such as fractionation by ethanol or fractionation by ammonium sulfate, a filtration by membrane, etc. and, in the removal of virus from a protein solution, a method of filtering by membrane which does not result in the denaturation of protein is attracting public attention.
On the other hand, in the steps for the manufacture of biopharmaceuticals and blood products, protein which is a useful ingredient should be efficiently permeated and recovered in view of the productivity. However, when the object for the separation and removal is a small-sized virus such as parvovirus, it has been difficult to simultaneously satisfy both of the removing characteristic for virus and the permeating characteristic for useful protein.
In the patent document 1, there is a disclosure for a hydrophilic porous membrane where the relation among an average permeability during 5 minutes from the initiation of the filtration (globulin permeability A), an average permeability during 5 minutes since the stage being elapsed 55 minutes from the initiation of the filtration (globulin permeability B), and the maximum pore size when 3 wt % bovine immunoglobulin where the percentage of the monomer is not less than 80 wt % is subjected to a low-pressure filtration at 0.3 MPa is expressed in terms of parameters. The constituent features of this membrane are as follows.
(1) Maximum pore size 10 to 100 nm
(2) Globulin permeability A>0.015×maximum pore size (nm)2.75 
(3) Globulin permeability B/globulin permeability A>0.2
Now, as mentioned in lines 21 to 27, page 3 of the specification, the requirement (1) merely mentions the pore size which is necessitated for the removal of infectious virus. The requirement (2) demands that the globulin permeability A is more than the value calculated from the maximum pore size of the micropore and, since it is obvious in a membrane for a purpose of removing the virus from a protein solution that the more the permeability for the protein solution, the better, it merely mentions the aimed characteristics. The requirement (3) demands that the permeability for a protein solution does not lower with elapse of time and that is also a mere description for the aimed characteristics which is demanded in the membrane where the removal of virus from a protein solution is a target. Besides the above, there are descriptions in subclaims for a hydrophilic porous membrane where the logarithmic removal rate to porcine parvovirus is 3 or more, for a hydrophilic microporous membrane where the accumulated transmission amount during 3 hours from the initiation of the filtration when 3 wt % bovine immunoglobulin where the ratio of the monomer is not less than 80 wt % is subjected to a low-pressure filtration at 0.3 MPa is not less than 50 liters/m2, etc. However, they merely mention the aimed characteristics of the membrane for a purpose of removal of virus from a protein solution where the virus is efficiently removed and the transmission amount of the protein solution is high and they do not give useful and specific information for an object of obtaining a membrane having a high transmission of protein and a high removal of virus.
There is also a disclosure for a finely porous membrane which has a coarse and big structure layer having a big porosity and a tight layer having a small porosity. To begin with however, the discussion made therein is substantially for a hollow fiber membrane made of poly(vinylidene fluoride) (hereinafter, it will be abbreviated as PVDF) which is apt to form a homogeneous structure by means of heat-induced phase separation. Thus, it is difficult to directly apply such an art, for example, to a raw material such as a polysulfone type resin which has been widely used as a material for the hemodialysis membrane due to its high water transmission ability.
The patent document 2 discloses a microporous membrane which has a coarse structure layer having a big porosity and a tight layer having a small porosity but, again, the thing substantially predicted as a material herein is PVDF. PVDF is excellent in terms of physical strength but, on the other hand, since it is a hydrophobic material, it is apt to result in adsorption of protein, etc. and also in staining and blocking of the membrane whereupon the filtration rate quickly lowers. In order to improve such an undesirable characteristics, it is necessary to make the membrane hydrophilic but, generally, a membrane where PVDF is a material is to be modified to a hydrophilic one by means of post-treatment after preparing the membrane. Thus, as compared with the polysulfone resin where it is usual to make into the membrane in a blended state with hydrophilic polymer, there is a disadvantage that troublesome manufacturing steps are resulted.
The patent document 3 discloses an ultrafiltration membrane for retaining the virus having the initial LRV of at least 4.0 to PhiX 174 where the surface is made hydrophilic with hydroxyalkyl cellulose. In the art disclosed therein, hydrophilization is conducted by a specific hydrophilic polymer and is lacking in broad applicability. Although a blend of polysulfone, etc. with a hydrophilic polymer such as polyvinylpyrrolidone is also exemplified, a hydrophilizing treatment using hydroxyalkyl cellulose is still inevitable. Further, although a hollow fiber type is allowed as well, a flat membrane type is substantially predicted and there is no sufficient explanation for preparing a hollow fiber membrane type.
The patent documents 4 and 5 disclose a method for removal of virus using a porous polymer where the ratio (Jp/Jw) of the permeability for a 5% by weight aqueous solution of human serum albumin (Jp) to the permeability for pure water (Jw) is not less than 1/50. In the patent documents 4 and 5, the facts that an inhibition coefficient of coliphage øX174 is not less than 2 and that an inhibition coefficient of gold colloid of 30 nm particle size is not less than 1 were mentioned as the constituent features, respectively. Anyway however, in the membrane characteristics mentioned therein merely stipulate the lowest limit of aimed characteristics as a membrane for a purpose of removal of virus from a protein solution. Thus, there is given no useful and specific information therein for the target of preparing a membrane where protein is highly transmitted and virus is highly removed. Moreover, the main membrane disclosed therein uses cellulose as a material and, since its strength in a state of being wet by water is low, it is difficult to highly set the pressure to be applied for filtration and it is not possible to achieve a high permeability.
The patent document 6 discloses a macromolecular porous hollow fiber membrane having such a pore structure that, from the inner wall surface to the inside wall, the in-plane void rate decreases initially and, after at least one minimum area, it increases again in the outer wall area and also discloses a method for removal of virus where an aqueous solution of protein is filtered using said membrane. When the membrane structure disclosed herein is briefly mentioned, it is a hollow fiber membrane where the pore size of the membrane wall becomes in the order of rough-dense-rough in the membrane thickness direction. It has been said to be suitable for the removal of virus in high efficiency and the recovery of protein in high transmission efficiency without denaturing protein that there are such an inclined structure and the specific average pore size. Although various macromolecular substances are exemplified as the material, it is substantially an art using regenerated cellulose and it is difficult to widely develop the art disclosed therein for many materials. Further, the disadvantages of a cellulose material were as mentioned already.
The non-patent documents 4 and 5 report that aggregation of protein is resolved by addition of a salt or by a treatment with DNAse, whereby the transmission rate of protein and the permeability for protein solution are enhanced. Although it is able to be well predicted that the salt concentration affects the existing state of protein and that the transmission efficiency is enhanced as such, no attention is paid for the fact that the salt concentration affects the interaction of membrane surface with protein whereby the transmission efficiency is increased or decreased.